The systems conventionally used in amplitude radio direction-finding (DF) are systems having crossed loops or systems having parallel loops distributed around the mast, where the latter systems present the advantage over crossed loop systems of making it possible to obtain considerable improvements in terms of the amount of error associated with readings, particularly when the direction of incidence of the electromagnetic wave to be measured coincides with the axis of the ship. As a general rule, crossed loop systems and parallel loop systems include an omnidirectional reference antenna which is a dipole antenna carried by the mast, which extends along the axis said mast, extending it upwards.
The accuracy of such DF systems is very highly affected in the high frequency (HF) range by reflections on obstacles placed in the vicinity of the antennas, and particularly when the system is used on board a ship, by reflections from the superstructure of the ship, its masts and its transmitter antennas.